In general, in designing a zoom lens, providing a negative refractive power to the object-side lens group has the benefit that a wide-angle of view is facilitated while also providing a long back focus for the insertion of necessary optical components between the zoom lens and the image plane. On the other hand, especially in a two-group lens construction where the lens groups are of negative and positive refractive power in order from the object side, the zoom lens system becomes asymmetric and aberration variations due to zooming become large, making it difficult to obtain favorable correction of aberrations across the entire range of zoom.
Examples of conventional two-group zoom lenses wherein the front lens group has negative refractive power and the rear lens group has positive refractive power are disclosed in Japanese Laid Open Patent Applications H9-171139 and H11-344670.
Although the lens system described in Japanese Laid Open Patent Application H9-171139 has an image angle of 100 degrees or more and thus is a wide-angle lens system, the second lens group has seven lens elements and the entire zoom lens includes as many as eleven lens elements, making it difficult to miniaturize the lens. On the other hand, although the lens system described in Japanese Laid Open Patent Application H11-344670 is easily miniaturized due to having only three lens elements in the first lens group and only eight lens elements overall, it is more difficult to favorably correct the various aberrations since there is less flexibility of design the fewer the number of lens elements. Further, because there are only two lens elements which provide the negative refractive power required in the front lens group, the curvatures of these lens elements must be relatively strong, causing the manufacturing costs for these lens elements to increase.
The present invention is a zoom lens having an image angle 2xcfx89 of 100 degrees or more at the wide-angle end, and is especially useful in monitoring cameras. Also, the present invention may be used in various other kinds of video cameras and electronic still cameras which use a solid-state image detector, such as a CCD array, to record the image.
The object of the present invention is to provide a compact, wide-angle zoom lens which favorably corrects aberrations and is easy to manufacture while providing an angle of view at the wide-angle end of 100 degrees or more.